This invention relates to an apparatus for the coating of continuous fiber tow. It also relates to a method for coating continuous fiber tow.
Composite materials are widely known and widely used. By combining a polymer with another material, such as glass, carbon, another polymer, or the like, it is possible to obtain unique combinations or levels of properties. Similarly, by combining a metal or glass with selected fibers, it is possible to obtain unique combinations or levels of properties. Advanced composites have evolved as a class of structural materials as a result of the development of high-modulus, high-strength, low-density reinforcing fibers.
The presence of a carbon interlayer along the fiber-matrix interface has been shown to be responsible for the high toughness and strain to failure of Nicalone.RTM. (SiC fiber)/lithium aluminosilicate glass composites and Nicalon.RTM./Ba-Si-AI oxynitride glass composites. However, these composites are not viable for high temperature oxidizing environments. Such environments require oxidation resistant fibers, matrices and interlayers. One approach to fabricating a fiber-matrix interface is to introduce an interlayer as a fiber coating before the composite is densified. After densification, the interlayer chosen should cause crack deflection and fiber pullout similar to carbon interlayers, or should provide oxidation resistance for other interlayers.
Several types or combinations of interlayers are considered to be feasible, including microporous interlayers, reactive interlayers which lose volume, and interlayers with ductile particles. However, application of a coating, particularly a uniform coating, to continuous fibers and fiber tows can be difficult. Measurement of coating thickness can also be difficult.
Several techniques are known for applying coatings to continuous fibers. Fiber coating may be accomplished by passing the fibers through a container filled with a coating liquid, which container has one or more rollers or wheels to keep the fiber immersed in the liquid while coating. One disadvantage of this process is that the fibers must be bent around the roller(s) or wheel(s) and may sustain damage from bending or abrasion. Other disadvantages are that the fibers may be contaminated from contact with the wheel or roller and that fibers which do not tolerate a small bending radius are prone to breakage or require a very large wheel and container.
Coatings may also be applied by spraying. The primary disadvantage of this coating method is that spraying is a line of sight process, so coating thickness is dependent upon the angle at which the spray jet contacts the fiber. Other disadvantages are that spray jets tend to clog easily, the characteristics of the jet may change with time, making control of the process difficult, viscous coating solutions are difficult to apply as a spray, low viscosity solutions tend to run off the fiber before they are cured, and the spray may be shadowed from inner tow filaments by the outer tow filaments.
Fibers may be coated by passing same through a container having a gasket which seals around the moving fiber and prevents coating liquid from flowing out. The disadvantages of this method are that the fiber surface may be contaminated or abraded by contact with the gasket, and fibers having irregular cross-sections or multifilament fibers or tows tend to get caught along irregularities or at broken fibers in gaskets tight enough to prevent leakage of the liquid.
Hay et al, U.S. Pat. No. 5,217,533, disclose an apparatus and method for coating continuous fibers in which a continuous fiber is transported through a coating composition container having an orifice in the bottom. Excess coating composition flowing out of the container is deflected from the fiber. In a preferred mode, the fiber does not contact the coating apparatus. The coating on the fiber may be cured immediately after the coating step.
Hay, U.S. Pat. No. 5,164,229, discloses an apparatus and method for coating continuous tow in which a continuous tow is likewise transported through a coating composition container having an orifice in the bottom. Excess coating composition flowing out of the container is deflected from the tow. According to this method, the coated tow is transported through a liquid which is immiscible with the coating composition and which acts to remove excess coating composition from between the individual fibers of the tow.
While both of these patents represent valuable advances in the state of the art, each of the methods has one or more of the following disadvantages:
(a) The coating liquid is recirculated directly into the coating vessel. The pumping rate has to be slightly faster than the rate at which the coating liquid leaves the coating vessel to insure that the coating liquid is maintained at a constant level in the coating vessel, so air is also pumped into the coating vessel. This can cause the coating liquid to froth, which in turn causes nonuniform fiber coating. PA1 (b) The coater can only be used under conditions where the fiber and coating are stable in air. Oxidation prone coatings and fibers can not be processed at high temperatures. This is a serious disadvantage for ceramic matrix composites, since one of the most promising coating concepts involves applying carbon as a fugitive phase. In the fugitive phase concept, carbon, or a carbon-oxide mixture is applied to the fiber. The fiber is incorporated into a composite and densified under reducing conditions where the carbon is stable. The dense composite is then heat-treated in air to oxidize the carbon away, leaving pores or a gap along the fiber matrix interface. This porous interface, or gap, deflects cracks and promotes fiber pullout, making the composite flaw tolerant. PA1 (c) While the coater can be used for most commercially available fiber tows, certain tow has very small filament diameters, many free ends, and some limp strands that do not support full fiber tension. If a free end or limp strand were to hang up in the orifice at the bottom of the coater, it will unravel the tow, rather than break off, because of the high strength and fine diameter of the filaments. This creates a larger obstruction in the orifice for other free ends and limp strands. Eventually the entire orifice plugs up and the fiber pulls apart. In principle this problem could be easily solved by making a much larger diameter orifice, but in practice a larger orifice is impractical because of the very high liquid pumping rates required and the consequent liquid foaming. PA1 (d) A further limitation was that only one coating can be applied at a time. If two different coatings are desired, two separate runs through the coater are required.
It is an object of the present invention to provide an improved apparatus for coating continuous fiber tow.
It is a further object of the present invention to provide an improved method for coating continuous fiber tow.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.